Is it possible that the same molecule (protein, lipid or lipoprotein) can have both postive and negative actions on the same cellular event? Any suggestions, examples to this question.

many cytokines have activation and repressive functions.

The same could be said for some xenobiotics. Ritonavir is a good example of a CYP3A4 inducer and inhibitor.

A molecule that I worked with - CTLA4 - an inducible T cell surface receptor that binds B7.1 and B7.2, was widely reported for over a decade as solely an inhibitor of t cell activation. However, some recent work, including ours, has now demonstrated in multiple systems that it can act both ways depending upon the T cell activation conditions.
I guess biological networks are so complex that most anything conceivable is possible - its only a matter of time before it is discovered! :)

Fas/fas laigand is another example. The strength of the signal transpmitted through fas (CD95) depneds if the ligand is membrane bound or soluble. For instance, activated T-cell exposed to membrane bound fas ligand are induced to apoptosis. Soluable fas ligand is not strong enough and can actually interfere with the membrane boudn signalling, preventing apoptosis. Uisning anti-fas antibodies (insted of the natural ligand) we can induce apoptosis using an IgM (mulei-valent) antibody, but anti-fas IgG won't cause cell death and can prevent cell death much like soluble fas ligand.

Another example is the TNF receptor which can activate both an apoptotic pathway (through caspase-8 -> Bid cleavage -> Bax activation) and a survival pathway (NF kappa-B activation), sometimes simultaneously.